Optical isolators are devices used in fiber optic transmission systems. They are important devices in high-speed systems including optical communications systems because they block light from traveling in the wrong direction through the fiber, i.e., they block back reflections. Back reflections may occur within a fiber when the transmitted light hits a point of discontinuity. Discontinuities may occur for a number of reasons, such as when there is a change in the refractive indices of interfacing materials, perturbations along the transmission path, or misalignment of the cores of optical fibers being connected. Back reflections present a serious problem in optical fiber systems as they introduce undesirable noise components into the signal, degrade performance, and can corrupt the transmission source, which typically is a laser.
A prior art optical isolator device is schematically illustrated in FIG. 1, comprising a pair of polarizers 12, 22, and a Faraday rotator 14. Light travels in the forward direction from optical fiber 8 having an input port 10, through the first polarizer 12, the rotator 14, and the second polarizer 22, following arrows "F" of FIG. 1. Light leaves the optical fiber 8 unpolarized or with an arbitrary polarization, exemplified by encircled arrow 10a. After passing through the first polarizer 12, the light becomes polarized specifically, the first polarizer blocking light of horizontal polarization and transmitting vertically polarized light (exemplified by the encircled arrow 12a). The vertically polarized light then enters the Faraday rotator 14. With the rotator, the polarization is rotated 45 degrees, shown here in the clockwise direction, exemplified by encircled arrow 14a. A Faraday rotator 14 typically is fabricated with a plate of yttrium iron garnet (YIG) crystal surrounded by a magnet for applying a magnetic field and making the crystal optically active. Upon exiting the rotator, the light then travels straight through the second polarizer 22 (also called the "analyzer"), which is orientated at a 45 degree angle to the first, with essentially no losses (e.g., 22a). The transmitted light exits to the output port 20, and onto other parts of the optical communications system. For simplicity, a focusing means used in the device of FIG. 1 is not shown.
With this isolator, light will not be back reflected to input port 10 because when the light is polarized in the same way in the reverse direction, it will not be transmitted through the first polarizer 12. In particular, light traveling in the backward direction, following arrows B, will exit fiber 20 unpolarized (e.g., 20a). Light passes through the second polarizer 22 at a 45.degree. angle (e.g., 22b), and then it will pass through the Faraday rotator 14, which rotates the light by another 45 degrees (e.g., 14b). The Faraday rotator is a non-reciprocal rotating device, such that the light passing through it in the reverse direction is rotated in the same (here clockwise) direction as light passing through it in the forward direction. Thus, when traveling in reverse, the light leaving the Faraday rotator is rotated 90 degrees relative to the vertical transmission path of the first polarizer 12 (hence it is horizontally polarized) and is therefore blocked, since the first polarizer transmits only vertically polarized light. Thus, the optical isolator 10 allows light traveling in the forward direction and having a specific polarization to be transmitted but blocks all polarizations of light traveling in the reverse direction.
As can be seen, these traditional isolators are polarization-selective. However, the transmission system itself may cause uncontrollable changes in polarization, thereby altering the polarization of the light entering the system and reducing the utility of these isolators. Thus, efforts have been made to develop polarization-independent optical isolators. These efforts have concentrated on using birefringent media in place of the polarizers to split the light signal into two orthogonally-polarized light paths. With these devices, the light is passed through two birefringent plates. In the forward direction, the light rays are split by one plate and then recombined by the second plate; however, in the reverse direction, the light rays after being split by the first plate are either not recombined or are further split by the second plate, so that the rays are physically separated. The separation of the rays causes the signal to be intercepted. See, e.g., U.S. Pat. No. 5,033,830, issued Jul. 23, 1991 to Jameson, titled "Polarization Independent Optical Isolator," assigned to the assignee herein, which is hereby incorporated by reference; and U.S. Pat. No. 5,044,713, issued Sept. 3, 1991 to Mozer, et al., titled "Optical Isolator," incorporated herein by reference.
A difficulty with these isolators is that the thicknesses of the birefringent plates may need to be precisely controlled; that is, the thicknesses of the two plates must be essentially identical because any difference will affect the accuracy of alignment of the two light paths relative to one another. This has been addressed by using a single birefringent plate with a reflecting means, as in the Jameson patent, so that the same plate both splits and recombines the signal.
A few designs exist for reflexive isolators, as in the Jameson patent. With these designs, the input and output waveguides are on the same side of the device; light emerging from one port is redirected by a reflector back into the end of another fiber on the same side. Another approach in addressing difficulties related to the thicknesses of birefringent plates is to use an electric field to control the refractive index of the birefringent medium, e.g., polymethyl methacrylate (PMMA), U.S. Pat. No. 5,191,467 issued to Kapany et al. titled "Fiber Optic Isolator and Amplifier" (hereinafter the "Kapany patent"), incorporated herein by reference.
As may be appreciated, those concerned with the development of optical communications systems continually search for new components and designs, including new designs for polarization-independent optical isolators. As optical communications systems become more advanced, there is growing interest in improving devices for optical circuits and for integrating them. The instant invention provides a new structure for a polarization-independent isolator which may be used as a reflexive isolator or to couple integrated optical circuits. Further advantages may appear more fully upon considering the description given below.